Is the conversion of ATP to ADP spontaneous?

Yes, the conversion of ATP to ADP is spontaneous. It doesn’t require energy to take place.  

ATP is a highly unstable molecule with three phosphate groups. The last two phosphate groups are attached by a high-energy bond known as the pyrophosphate bond. If ATP is not used immediately to perform work, it spontaneously detaches its third phosphate group, dissociating into ADP + Pi. The breaking of the high-energy pyrophosphate bond releases a large amount of free energy, which is lost as heat. Cells use a strategy called energy coupling to harness the energy within the ATP bonds.